a) Field of the Invention
The present invention relates to a method for determining the strength of a join between a ceramic and a non-ceramic.
b) Description of the Related Art
Ceramics have excellent material characteristics in hardness, abrasion resistance, corrosion resistance and the like, and are expected to find utility as principal equipment materials in many fields from now on. These fields include some where ceramics may be used as discrete members, parts or components (hereinafter collectively called "members"), but ceramics are also expected to be employed in forms where they are joined with one or more other members in many other fields. Especially in the field of mechanical or structural members, ceramics will be used in a form where they are joined with a metal.
A ceramic is different in coefficient of thermal expansion and modulus of elasticity from a metal, so that when they are joined together, a residual stress is produced in the ceramic and the strength of the resulting body so joined is governed by the residual stress. To form a good joined body, it is therefore indispensable to ensure joining results in minimal residual stress. A variety of joining methods have hence been attempted. FIG. 1 is a side view of a joined ceramic-Kovar (trade mark) body, illustrating one of such joining methods. In the drawing, there are depicted a ceramic 1, Kovar 2, and an intermediate nickel material 3. It has been reported that the joining of the ceramic 1 and Kovar 2 by using the intermediate material as shown in the drawing leads to increased strength of the ceramic 1 at the join compared with their direct joining and the strength varies depending on the thickness of the intermediate nickel layer 3.
The quality of each joining method for a ceramic and a non-ceramic is determined by measuring the residual stress. Examples of this measurement include a method for calculating such residual stress by using the finite element method, conventional X-ray diffraction, IF (indentation fracture) technique, laser spectroscopy, etc.
However, the above calculation of residual stress in a ceramic by the infinite element method is complex and time-consuming, so that this measurement method cannot be used for the inspection of dispersion or the like of actual products. On the other hand, X-ray diffraction, IF technique and laser spectroscopy involve one or more problems as will be discussed below.
Because a ceramic is a sintered material, the residual stress produced at a join between the ceramic and a non-ceramic has a more complex distribution compared with the residual stress produced at a join between non-ceramics. Further, the residual stress in the former case has a stronger tendency to concentrate on the outer surface of the join, whereby the residual stress changes abruptly depending on the position. With reference to a drawing, a description will next be made of one example of stress distribution which occurs in a typical ceramic/non-ceramic joined body.
FIG. 2 is a stress distribution diagram of a joined ceramic-metal body which makes use of an intermediate nickel material. In this diagram, Si.sub.3 N.sub.4 and (W/Ni)/Fe-30Cr are used as the ceramic and the metal, respectively. Distances (mm) from the joined end surface of the ceramic on the side of the ceramic are plotted along the axis of abscissas, while stresses (MPa) are plotted along the axis of ordinates. Curves A, B and C correspond respectively to joined bodies in which the thicknesses of the respective intermediate nickel layers are 2.0 mm, 1.25 mm and 0.5 mm. The following can be envisaged from the diagram. In the joined body C, tensile stress exists throughout the ceramic. In particular, tensile stress reaches as great as 600 MPa around the interface between the ceramic and metal. In the joined body A on the other hand, tensile stress is developed at the ceramic-metal interface but this tensile stress decreases with the distance from the interface. After compressive stress is developed once, tensile stress is produced again. This tensile strength reaches a maximum value and then decreases gradually. Turning next to the joined body B, the distribution pattern of stress is similar to that of the joined body A but the stress is not greater than 200 MPa.
As is shown in the stress distribution diagram described above, the distance-dependent stress variations of the joined body C are relatively gentle but those of the joined bodies A and B are complex and steep. Accordingly, the measurement of stress in a small region in the order of .mu.m is apparently indispensable for the determination of accurate stress distribution.
The conventional measurement methods, namely, X-ray diffraction, IF technique and laser spectroscopy are however unable to conduct measurement in such a small region. As a consequence, measurement by such a conventional method covers a substantially wide region. The measurement therefore provides, as the result, an average value of residual stress across a wide region. It is by no means possible to obtain an accurate stress distribution diagram. Moreover, breakage of a ceramic takes place at once from the point where the residual stress is maximum. This maximum value of residual stress may be overlooked as long as such an average value as described above is relied upon, leading to the problem that the resulting stress distribution diagram is extremely inconvenient for practical applications.